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Schmitt Trigger Synthesizer

For this circuit, you may need to connect the audio jack to a guitar amp. A regular stereo out may not have enough gain to hear the signal.

A Schmitt trigger is a type of threshold circuit with positive feedback. The circuit is named "trigger" because the output retains its value until the input changes sufficiently to trigger a change. A Schmitt trigger is a bistable multivibrator; in its inverting configuration it can be used as an oscillator. The integrated circuit chip we are using is called a hex Schmitt trigger because it has six inverters on one chip. For this exercise, you can use 74C14 or CD40106, they are both Hex Schmitt Triggers.

Single inverter

pin 14 goes to voltage source

pin 7 goes to ground

R1 = 10k (resistor between pin 1 and pin 2)

C1 = .1uF (capacitor between pin 1 and ground)

The hot tip of the audio jack connects to pin 2, which is the OUTput signal

The sleeve of the audio jack connects to ground

Step 1: ​Set Up One Oscillator, and Connect It So We Can Hear It

Step 2: Replace the Resistor With a Photoresistor

Step 3: Replace the Resistor With a Potentiometer

Step 4: Multimeter: Measure the Resistance of the Photoresistor and Potentiometer

Write down the range of resistance for your potentiometer and photoresistor.

Two inverters

pin 14 goes to voltage source

pin 7 goes to ground

R1 = 10k (resistor between pin 1 and pin 2)

R2 = 10k (resistor between pin 3 and pin 4)

C1 = .1uF (capacitor between pin 1 and ground)

C2 = .1uF (capacitor between pin 3 and ground)

R3 = 10k (resistor between pin 2 and OUT)

R4 = 10k (resistor between pin 4 and OUT)

The hot tip of the audio jack connects to OUT

The sleeve of the audio jack connects to ground

Step 5: ​Use Two Inverters

To connect multple inverters to the same audio output, send each signal through a 10k resistor which all terminate at the hot tip of the audio jack. To play with the signal, could substitute R1 and/or R2 for variable resistors like a potentiometer or photoresistor.

Step 6: Use Three Inverters

Step 7: Use Three Inverters

This time use a 10k resistor for inverter #1, a potentiometer for inverter #2, and a photoresistor for inverter #3.